The present invention relates generally to pulse generating means for generating a train of pulses which varies between two adjustable frequency limits, and more particularly to means for providing a ratio of substantial magnitude between upper and lower frequency limits and for maintaining said predetermined frequency limits when adjusted to the desired levels regardless of normal variations in components comprising a portion of the pulse generating means. Pulse trains of the type described above have many uses, one, for example, being in controlling stepping motors which normally function at one of two operating speeds and which are required to shift smoothly from one of said operating speeds to the other. Such stepping motors find utility in many applications, such as driving operation sequence instruction display films employed in various types of business machines.
A number of timing devices are now available on the general market which are capable of generating a variable frequency pulse train for use in such applications as driving stepping motors, for example. These devices are attractive in many respects, in that they are monolithic in construction, small in size and inexpensive in price. One problem with such timing devices is that the range between the upper and lower frequency limits is not great, which renders them unsuitable for some applications. Such range cannot normally readily be varied in these devices. Another problem is that the production of such timing devices in large quantities may result in variations in components within the devices, so that they will not operate in a uniform manner from device to device. Such uniformity of operation is essential when the timing devices are incorporated in products which are produced in substantial quantities. The necessary uniformity of operation can be achieved in such cases only by careful and precise testing of timing devices and/or by the incorporation in the product using the timing devices of complex adjusting circuits requiring individual testing and adjustment for each individual product in which the timer is used.